Fire detector cable



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FIRE DETECTOR CABLE Filed Nov. 1'7, 1955 Elm 500 TEMPERATURE PF.)

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Patented Aug. 19, 1958 FE DETECTOR QABLE Robert H. Postal, Clifton, N..l., assignor, by assignments, to McGraw-Edison (Company, Elgin, ill, acorporation of Delaware Application November 117, 1953, Serial No.392,565 5 Claims. (fil. Elli-63) This invention relates to fire detectorcables of the character described and claimed in the pending Kelly andPostal application Serial No. 241,992, filed August 15, 1951 (now PatentNo. 2,740,874, dated April 3, i356) and having common ownership with thepresent application. The invention relates particularly to improved firedetector cables of this character, and to novel and improved methods ofconstructing such cables and of preparing the temperature-responsivematerial thereof.

These fire detector cables use temperature-responsive materialscomprising the electronic non-stoichiometric oxidic semiconductorsi. e.,compounds with a deviatien from a simple stoichiometric ratio. Suchsemiconductors have temperature-responsive characteristics depending ontheir oxygen contents but are notably instable. However, in the firedetector cables of the aforesaid application, stability is achieved bycompacting the semiconductor in a sealed metal sheath of the cable undergreat pressure and by oxidizing the internal surfaces of the cable.

An object of. the present invention is to provide new and improvedtechniques in fabricating such cables, which enables cables havingpreselected temperature-resistance characteristics to be produced undercontrolled production methods.

Another object is to provide a new and improved fabricating technique bywhich cables can be produced economically under controlled conditions tohave any preselected resistivity within a wide range at a giventemperature.

Another object is to provide such improved fabricating technique whichcan be carried out accurately under mass production methods.

Another object is to provide a new and improved methed by whichelectronic non-stoichiometric oxidic semiconductors for the present typecable can be prepared to provide such cable with any given resistivitywithin a wide range of temperatures.

Another object is to provide fire detector cables having improvedstructural and operating characteristics, and especially greateruniformity of operating characteristics along each cable and greaterstability of operation under repeated cyclings through wide temperatureranges.

It has been the prevailing thought that electronic nonstoichiometricoxides have very limited practical application. For example, in thearticle Controlled-valency semi-conductors by E. l. W. Verway, P. W.Haaijman, F. C. Romeijn and G. W. van Oosterhout, Philips Res. Rep. 5,173-187, 1950, it is stated, page 176:

The application of non-stoechiometric compounds'as semi conductors inresearch and practice has rather severe limitations. One reason is apurely practical one, viz the circumstance that the preparation of ahomogeneous and stable semi-conductor with a reproducible value of thespecific resistance in this way is generally very difficult or evenvirtually impossible. Secondly the deviation from simple stoechiometrythat can be realized is in most cases rather small and the possibilityof variations of the specific resistance accordingly restricted,

i have found that electronic non-stoichiometric semiconductive metaloxides can be prepared under controlled reproducible conditions for usein the present type cable to provide cables with very stable operationhaving widely different specific resistances. This preparation iscarried out by selecting, as initial ingredients, different oxideshaving substantially diiferent oxygen contents and by intimately mixingsuch oxides in predetermined proportions.

A further object of my invention is to provide a methad of producingfire detector cables under substantially invariable conditions whichenables the operating characteristics of the cables to be predeterminedon the basis of the metal oxide mix and the heat-treating operation.

These and other objects and features of my invention will be apparentfrom the following description and the appended claims.

In the description of my invention reference is had to the accompanyingdrawings, of which:

Figure l is a fractional cross sectional view of a fire detector cableincorporating my invention;

Figure 2 is a graph showing the resistancev variation of a cable at agiven temperature with variation in the oxygen content of thetemperature-responsive material using oxides of manganese; and

Figure 3 is a graph showing several resistance-temperaturecharacteristics of cables using temperature-responsive manganese-oxidematerials of different oxygen content. The present fire detector cablemay be geometrically the same as described, in the. aforementionedapplication. For example, it comprises a central metal wire it)constituting one electrode of the cable, a spaced surrounding metalsheath 11 constituting a second electrode and an interveningtemperature-responsive material 12, which comprises electronicsemiconductive metal oxide in its entirety or as its principal element.The temperature-responsive material is packed into the cable under greatpressure as by swaging the sheath to a reduced diameter after thematerial is loaded into the sheath. This temperature-responsive materialpreferably comprises the sole medium in the cable for holding thecentral wire centralized in relation to the sheath except for the use ofceramic beads 13 at the ends of the cable. The ends are closed air-tightby hermetic seals 14 each comprising a glass bead 15 fuzed to outer andinner tubing sections 16 and 17. These tubing sections are telescopedonto the sheath and wire respectively and the ends thereof are securedairtight to the sheath and wire by silver soldering at 18 and 19respectively.

The sheath and center wire must be capable of withstanding extremelyhigh temperatures running beyond 2000 F. when engine oil fires onaircraft are to be detected. The sheath should also be ductile to enablethe swaging aforementioned and to enable sharp bending of the finishedcable in meeting particular installation requirements. Nickel-iron alloyof about 42% nickel and the remainder iron admirably fulfils theserequirements. As for the center wire, there is required a metal whichwill also acquire a low-resistance contact with the semiconductive metaloxide under the conditions of fabrication of the cable. Except for thenoble metals--which are hardly acceptable in most practical applicationsbecause of their high costthe run of such metals will form an oxidizedsurface layer in an oxygen atmosphere at flame temperatures. Arequirement of such oxidizable. metals is therefore not only that theywithstand flame temperatures without melting but also that the oxidesthereof have low electrical resistivity and the capability of bondingphysically with the metal oxide of the temperature-responsive materialby mutual interlacing of the 3 oxides at flame temperatures. It has beenfound that substantially pure iron wire fulfils admirably all of theserequirements.

By way of typical example, the sheath may have an outside diameter of.070" and a wall thickness of .011, and the center wire may have adiameter of .020, leaving an intervening layer of temperature-responsivematerial of .012" thickness. Wide variations in these particulardimensions may of course be made.

As aforementioned and as will be understood from the Kelly et al. PatentNo. 2,740,874, by using an airtight metal-sheathed cable having oxidizedinternal surfaces and by densely compacting or otherwise consolidatingthe temperature-responsive material in the cable, it becomes possible toobtain stable reproducible temperature-responsive characteristics withthe use of electronic semiconductive metal oxides that are generallyrecognized as being very instable. It is not fully understood why stableoperation is achieved from such cable construction, but it is believedthat the heavy compacting of the semiconductor precludes gaseousdiffusion therethrough and that this is an important factor. Each of thesemiconductors usable for the present purpose has atemperature-resistance characteristic dependent upon its oxygen content,such that the resistance at a given temperature increases as the oxygencontent is reduced. Also, each of the semiconductors has the propertythat it tends to release oxygen as gas when heated under given pressureconditions to certain temperature regions, the temperature region beinghigher as the oxygen content of the semiconductor is smaller, and viceversa. Although this temperature region may not fall for certainsemiconductors in the operate temperature range of the cable at which analarm is to be sounded or a firecombative apparatus is to be put intooperation-thc operate temperature range being typically between 200 F.and 1000 F.it is to be recognized that in practice the cables aresubjected to much higher temperaturesas high as 2200 F. and more whendetecting oil fires on aircraftbefore the fire can be coped with. Thesehigher temperatures exceed the abovementioned gas-releasing temperatureregions of the usable electronic oxidic semiconductors with the resultthat oxygen gas is released during each fire-detecting operation. Whengaseous diffusion through the semiconductor is precluded, the oxygenreleased from each elemental portion as the cable is heated is retainedin intimate association with that portion for total recombinationtherewith as the cable is cooled to assure that the semiconductor willreturn to its original state and have again the same resistivity aftereach heating-cooling cycle. The oxides which are usable for the presenttype of cable are those of the metals: cobalt, manganese, chrome, nickeland copper.

The present invention resides particularly in a new and improvedfabricating technique which enables cables to he produced undercontrolled conditions having a given resistivity within a widetemperature range of operate temperatures. Also the cables so producedhave improved construction and greater operating stability and greateruniformity.

It has been found that electronic non-stoichiometric oxidicsemiconductors can be produced to have different prescribed oxygencontents and that these oxides can be intimately mixed in preselectedproportions to provide improved temperature-responsive materials for thepresent type cable, to enable cables to be produced having an operateresistance, say 100 ohms for a 50 length of cable of the dimensionsaforementioned, at different operate temperatures in a rangeapproximating between 200 F. and 1000 F. Accordingly I am able toproduce a family of cables using the same basic metal oxide to meetspecific requirements of different fire-detection applications. By wayof preferred example, I herein next illustrate my in vention inconnection with the use of oxides of manganese, it being understood thatthe same technique applies similarly to the other oxides abovementioned.

Oxides of manganese can be produced in various forms havingwidely-different oxygen content ranging from manganous oxide (MnO) tothe higher forms such as MnO including the non-stoichiometricintermediate forms. By experiment it has been found that a 50' length ofcable of the dimensions aforementioned, using a substantially pure ironcenter wire, has a resistance of ohms and an operate temperature ofapproximately 535 F. when the starting oxide of manganese which isloaded into the cable has the composition MnO To obtain this particularcomposition I mix two readily-preparable forms of oxide of manganesehaving respectively lesser and greater percentage oxygen content-i. e.,ratio of oxygen to manganese-than that of MnO Preferably, I use a nearform of manganous oxide prepared by firing manganese dioxide at 1000 C.in a pure hydrogen atmosphere for one hour, and a second oxide ofmanganese obtained by firing such manganous oxide in air at 1000 C. forabout one hour. The first oxide so obtained measuser typically bychemical analysis to be MnO and the air-fired oxide measures to be MnOSince both of these oxides are prepared in definite atmospheres-the onebeing prepared in air for convenience-repeated production runs willresult in uniform materials. It may be parenthetically noted that onfiring the manganese dioxide in hydrogen the color thereof changes fromblack to light green and upon subsequently firing the green oxide in airthe color changes to brown.

These green and brown oxides are micropulverized and run through a325-mesh screen, and are then mixed in such proportions as to give acomposition having an oxygen content represented by the formula MHOLUDQ.The formula for compacting the specific proportions of the two oxidesstems from the equation that the content of oxygen by weight in thegreen material plus the content of oxygen by weight in the brownmaterial must equal the content of oxygen by weight in the resultingmix. For example, if X represents the parts by weight of the greenmaterial then (100-X) represents the parts by weight of the brownmaterial. The oxygen content of X parts of the green material (Mno willbe inoz of (IOU-X) parts of the brown material (MHOL25I1 will be and of100 parts of the final mix will be 1.009 umc On equating andsubstituting the atomic weights for oxygen and manganese and solving theequation, it will be found, for example, that 96.975 parts of the greenoxide 3.025 parts of the brown oxide by weight are required to give thedesired mix.

Having prepared the desired oxide mix, the same is mixed with anextruding lubricant such as Veegum, described in the Kelly et al. PatentNo. 2,740,874, and with 10% by Weight of water, and the mix is thenextruded under great pressure on the iron center wire with a diameter of.100", the wire being first roughened to prevent oxide slippage. Thisextrusion is dried in air for about twenty-four hours and is then passedthrough an extrusion furnace for about fifteen minutes at 1500 F. in aninert atmosphere which may be commercially pure nitrogen. Since theparticular oxide of manganese here considered has a low oxygen contentit does not release oxygen in such nitrogen atmosphere at thistemperature. The treatment does, however, render the mix firmly adherentto the center wire. Directly after this treatment the extrusion isthreaded into a clean metal sheath such as above described, havinginitially such inside diameter as to clear the extrusion by about .005.The sheath is drawn next through dies to establish a firm contactbetween it and the extrusion, and then the sheath is swagedprogressively to a diameter of about .070" to compact themanganese-oxide mix under great pressure.

The swaged cable is then heat-treated at about 1000 C. for fifteen totwenty minutes in a reducing atmosphere of pure carbon monoxide and issealed at its ends to complete its construction. This final heattreatment is the same for all cables, the purposes thereof being: (1) toprovide the inside wall of the sheath and the surface of the center wirewith oxide films of the respective metals formed by oxygen from themetal oxide extrusion to thicknesses representing a substantially stablestate of oxi dation whereat substantially no further oxidation will takeplace in the practical use of the cables, and (2) to anneal the sheathfrom the brittle condition thereof resulting from the cold rolledswaging operations. In this final heat treatment, oxygen will diffusethrough the semiconductor to the metal sheath and center wire because oftheir greater afiinity for oxygen at the heat-treating temperature.Experimental analysis shows that whereas the composition of thesemiconductor was originally MnO it is MnO after the final heattreatment. Since the final heat treatment is substantially the same forall cables, the electrical resistivity of each cable may be determinedsubstantially on the basis of the oxygen content of the oxide mixtureloaded into the cable.

In the above example, the metal oxide mixture which is loaded into thecable is purposely prepared to have a greater amount of oxygenconcentration than is required in the end product by the amount ofoxygen lost to the sheath and center wire in the final heat-treatingoperation. If the sheath and center wire were made of substantiallynon-oxidizable metals such as the noble metals, the final heat treatmentwould not be necessary and no appreciable allowance for oxygen losswould have to be made.

In Figure 2 there is shown a graph, determined empirically, which showsthe resistances of 50' lengths of cable of the type and dimensionsabovementioned, at an operate temperature of 500 F., for starting oxidesof manganese having different oxygen content ranging from MnO to MnO Forexample, this graph shows that a cable using a starting oxide mixturewhose composition is MnOLogg will have approximately 200 ohms resistanceat 500 F. As shown by the graph of Figure 3, with particular referenceto curve A, the temperature at which this cable will have 100 ohms is535 F. Curve A of Figure 3 is determined experimentally. Once such curveis known, there may be readily drawn a family of such characteristicsfor cables having different oxide mixtures, since these characteristicsrun along each other with approximately equal spacing. For example, thegraph of Figure 2 shows that a 50' cable of the aforesaid dimensionswill have approximately 45 ohms at 500 F., When a starting oxide mix ofthe formula MnO is used. This establishes point P on graph 3. Theresistance vs. temperature curve for a cable with this particular oxidemixture will then be approximately as shown by curve B. This lattercable will thus have approximately 100 ohms resistance at a temperatureof about 460 F.

It will be understood that the examples herein specifically describedare intended to be illustrative and not limitative of my invention sincevariations in the temperature and other conditions and in the materialsmay be made without departure from the scope of my invention, which Iendeavor to express according to the fol' lowing claims,

I claim:

1. A resistance-type temperature-responsive cable of indefinitecontinuous length comprising an impervious metal sheath and a spacedmetal center wire having oxidized surfaces; means sealing said cable atthe ends; a temperature responsive material filling the space betweensaid sheath and center wire, said material comprising predominantly anelectronic non-stoichiometric oxidic semiconductor tending to releaseoxygen when heated and to recombine with available oxygen when cooledand having temperature-responsive characteristic depending on thepercentage oxygen content thereof, said semi-conductor being selectedfrom the group consisting of the oxides of Co, Mn, Cr, Ni and Cr, saidsemiconductor comprising a plurality of semiconductive metal oxides ofthe same metal, said oxides having different percentage oxycontents andbeing in a finely mixed pulverulent state compacted in said sheath intoan essentially solid mass substantially impervious to diffusion ofreleased oxygen gas therethrough when the cable is heated.

2. The temperature-responsive cable set forth in claim 1 wherein saidoxides have preselected lesser and greater percentage oxygen contentsand are predeterminately proportioned by weight to provide said materialwith a predetermined intermediate percentage oxygen content adapted toprovide said cable With a preselected resistivity at a presettemperature.

3. The temperature-responsive device set forth in claim 1 wherein saidsemiconductor comprises a mixture of substantially manganous oxide (MnO)and an oxide of manganese having a higher oxygen content.

4. A resistance-type fire-detection cable comprising a metal sheathhaving an inside wall of a metal oxidizable to form a semiconductiveoxide, a center wire of iron, an intervening temperature-responsivematerial comprising an electronic semiconductor of a type which has aresistivity at a given temperature depending on the oxygen contentthereof, said semiconductor being selected from the group consisting ofthe oxides of Co, Mn, Cr, Ni and Cu, said semiconductor being compactedin said sheath into a solid homogeneous mass substantially free of airspaces, the inside wall of said sheath and said center wire beingoxidized by oxygen from said semiconductor, said center wire beingbonded mechanically to said semiconductor by interlacing of oxides ofthe semiconductor and of the iron oxide on the center wire.

5. A temperature-responsive fire-detection cable comprising a metalsheath and a center wire having its surfaces in a stabilized state ofoxidation, an intervening temperature-responsive material comprising anelectronic non-stoichiometric oxidic semiconductor selected from thegroup consisting of the oxides of Co, Mn, Cr, Ni and Cu, saidsemiconductor releasing oxygen when heated and tending to recombine withavailable oxygen when cooled and having a resistivity which at a giventemperature varies according to the oxygen content thereof, saidsemiconductor being highly compacted in said sheath and saidsemiconductor comprising an intimate mixture of oxides having differentoxygen contents, said different oxides being proportioned to give saidcable a preset resistivity at a given temperature.

References Cited in the file of this patent UNITED STATES PATENTS

1. A RESISTANCE-TYPE TEMPERATURE-RESPONSIVE CABLE OF INDEFINITECONTINUOUS LENGTH COMPRISING AN IMPERVIOUS METAL SHEATH AND A SPACEDMETAL CENTER WIRE HAVING OXIDIZED SURFACES, MEANS SEALING SAID CABLE ATTHE ENDS, A TEMPERATURE-RESPONSIVE MATERIAL FILLING THE SPACE BETWEENSAID SHEATH AND CENTER WIRE, SAID MATERIAL COMPRISING PREDOMINANTLY ANELECTRONIC NON-STOICHIOMETRIC OXIDIC SEMICONDUCTOR TENDING TO RELEASEOXYGEN WHEN HEATED AND TO RECOMBINE WITH AVAILABLE OXYGEN WHEN COOLEDAND HAVING A TEMPERATURE-RESPONSIVE CHARACTERISTIC DEPENDING ON THEPERCENTAGE OXYGEN CONTENT THEREOF, SAID SEMI-CONDUCTOR BEING SELECTEDFROM THE GROUP CONSISTING OF THE OXIDES OF CO, MN, CR, NI AND CR, SAIDSEMICONDUCTOR COMPRISING A PLURALITY OF SEMICONDUCTIVE METAL OXIDES OFTHE SAME METAL, SAID OXIDES HAVING DIFFERENT PERCENTAGE OXYGEN CONTENTSAND BEING IN A FINELY MIXED PULVERULENT STATE COMPACTED IN SAID SHEATHINTO AN ESSENTIALLY SOLID MASS SUBSTANTIALLY IMPERVIOUS TO DIFFUSION OFRELEASED OXYGEN GAS THERETHROUGH WHEN THE CABLE IS HEATED.